In recent years, optical disks, optical cards, optical tapes, or the like have been proposed and under development as media for recording information optically. Of the above media, the optical disks have drawn attention, on which information can be recorded/reproduced with large capacity and high density.
One type of rewritable optical disks is a phase change optical disk. The phase change optical disk includes a recording film that is changed to be in the amorphous or crystalline state depending on thermal and cooling conditions by a laser beam. The amorphous and crystalline states can be reversed. The recording film has optical constants (refractive index and extinction coefficient) that differ in the amorphous and crystalline states. In the phase change optical disk, the two states are provided selectively on the recording film according to information signals so that the resultant optical change (a change in transmissivity or reflectivity) is used to record/reproduce the information signals.
To obtain the two states, information signals are recorded in the following manner. The recording film of an optical disk is irradiated with a laser beam in pulse form (with a power level of P1) focused by an optical head. When the temperature of the recording film is raised to exceed the melting point, the molten portion is cooled rapidly with the passage of the laser beam to form an amorphous mark. The power level P1 is called a recording power. For irradiation of the recording film with a focused laser beam (with a power level of P2, P2<P1) having an intensity that increases the recording film temperature to temperatures ranging from the crystallization temperature to the melting point, the irradiated portion of the recording film is crystallized. The power level P2 is called an erasing power.
In this manner, a recording pattern, including a mark of the amorphous area and a non-mark portion of the crystalline area (referred to as a space) that correspond to the information signals, is formed on a track of the optical disk. The information signal can be reproduced by utilizing a difference in the optical characteristics between the crystalline and amorphous areas.
Recently, a mark edge recording (also referred to as PWM recording) system is often used instead of a mark position recording (also referred to as PPM recording) system. In the mark position recording, information is represented by only the mark position itself. On the other hand, in the mark edge recording, it is represented by both the leading and trailing edges of a mark. Thus, the latter has the advantage of improving the recording linear density.
To achieve further improvement in the recording density, a multi-value recording method has been proposed, in which information of three or more values is recorded on a single mark. For example, JP 4 (1992)-209319 A discloses a method for recording multi-valued information by forming marks having different sizes with laser beam irradiation of at least three power levels.
However, the conventional multi-value recording method described above has a problem in that, particularly when used with the mark edge recording, information cannot be reproduced accurately due to an increase in the jitter of a reproduced signal. This is because the leading and trailing edges of an area in the mark where a width of the mark is constant and a space area where the width is zero (hereinafter, each of those areas is referred to as a unit recording area) are not formed at the precise positions.
For example, when a unit recording area with a large mark width is recorded according to multi-valued information, the recording power of a laser beam is increased, causing an increase in energy applied to a recording film. Since thermal diffusion in the recording film is isotropic substantially on the disk plane, the length of the unit recording area tends to be longer than a predetermined length. On the other hand, when a unit recording area with a small mark width is recorded, the recording power of the laser beam is reduced. Therefore, the length of the unit recording area tends to be shorter than a predetermined length. As a result, the leading and trailing edges of the unit recording area thus formed are shifted variously from the predetermined positions depending on the width thereof, which corresponds to a value of the multi-valued information. This prevents accurate reproduction of information that is represented by each edge of a unit recording area.